There is a method that creates a printed matter obtained by printing an image on a sheet having a duplication preventing function or attaches a seal to a printed matter for forgery prevention in order to prevent unfair duplication or forgery of a printed matter. However, in such a method, the cost of the sheet having the duplication preventing function or the seal for forgery prevention is too high.
Furthermore, there is also a technology (ground tint printing) that prints an image embedded with a character string as a background which appears if copied as a duplication prevention technology of a printed matter. However, in the ground tint printing, there is a problem that small characters are not seen or the background becomes an obstacle.